Spark needs a GUI dialog, which it could present when a user tries to login to a server with an invalid certificate. It should present why certificate is invalid (maybe it can be pulled from Smack error, but there should be a conversion of this error to a human readable text): self-signed, expired, not trusted authority, mismatching hostname. There should be a button to Proceed anyway, Cancel (which will not let Spark to connect). There should also be a checkbox to add this certificate to the exceptions list (when pressing Proceed button), so it won't ask again on a next login. There could be also a GUI to manage exceptions or just a button somewhere to wipe the list, but that could be added later.
When this GUI is implemented, we should disable by default Accept all certificates and Disable hostname verifications options.These options can stay for those, who are using Spark in a closed environment and think they are safe from certificate spoofing attacks, so they won't bother their users with additional dialogs.
If the certificate is perfectly fine on a first check, Spark shouldn't show any dialog for it. But it shouldn't add such certificate into the exception list. It should validate valid certificates every time it logins. And when a valid certificate becomes invalid (say, expires), then it should show a dialog for it.